A variety of power conversion devices capable of providing a variety of types and levels of power for a variety of different purposes are now available on the market. A number of these power conversion devices are designed to output three-phase, alternating current (AC) electrical power for use by three-phase AC machines and other devices. For example, in the field of electric motors and motor drives, a three-phase AC motor can be connected to a motor drive, which includes (and operates as) a power converter and provides three-phase AC electrical power to the motor in a controlled fashion. By controlling the currents (and voltages) applied to a given motor, the motor drive further is capable of controlling motor speed, torque and other motor performance characteristics.
One type of power converter that is employed in some such motor drives is a voltage source inverter (VSI). Referring to FIG. 1, one such Prior Art VSI power converter 2 is shown in schematic form. As shown, three-phase AC input (or supply) power received from a three-phase AC voltage source 4, which can be a utility/power line or other source and can be modeled (as shown) as three separate voltage sources Va, Vb and Vc, respectively. The input power is converted by the VSI power converter 2 into three-phase AC output power that is appropriate for a load 6, which in this example is a three-phase AC motor (e.g., an induction or synchronous motor), and can be modeled as three resistors 46 respectively in series with three inductors that represent three different windings of the motor.
The VSI power converter 2 operates by way of two stages, a first stage that is a rectifier 8 that converts the AC input power into direct current (DC) power, and a second stage that is an inverter 10 that converts the DC power into the three-phase AC output power of desired frequency and amplitude for the load 6. In the embodiment of FIG. 1, the rectifier 8 is a pulse width modulated (PWM) rectifier that employs first, second and third pairs of insulated gate bipolar transistors (IGBTs) 12, 14 and 16, respectively. The IGBTs of each pair 12, 14 and 16 are coupled in series with one another between first and second nodes 18 and 20, respectively. Additionally, first and second capacitors 22 and 24 are coupled in series between the first and second nodes 18 and 20.
Further, first, second and third nodes 26, 28 and 30 between the respective pairs of IGBTs 12, 14 and 16, respectively, are coupled to first, second and third input ports 27, 29 and 31, respectively, of the VSI power converter 2 by way of respective input filter inductors 44, the input ports respectively receiving power from the respective voltage sources. As for the inverter 10, it also is a PWM device that employs first, second and third pairs of IGBTs 32, 34 and 36, respectively. Again, the IGBTs of each pair 32, 34 and 36 are coupled in series with one another between the first and second nodes 18 and 20, in parallel with the capacitors 22, 24 and the IGBT pairs of the rectifier 8. Further, first, second and third additional nodes 38, 40 and 42, respectively, are formed between the IGBTs of each pair 32, 34 and 36 and are coupled to the load 6, such that first, second and third output power phases (shown in FIG. 1 as A, B, and C) are provided to the load.
Each of the IGBTs of each of the pairs 12, 14, 16, 32, 34 and 36 is controlled by way of a controller 33 (e.g., a microprocessor) to switch on and off at appropriate times such that substantially DC power (e.g., a DC voltage) appears across the first and second nodes 18, 20 and so that the DC power in turn is converted into desired AC output power provided as the phases A, B, and C. Discontinuities and/or ripple introduced due to the pulsing of the IGBTs are largely smoothed out by the operation of the capacitors 22, 24. Although the rectifier 8 and inverter 10 are shown to employ IGBTs, in alternate embodiments the rectifier and/or inverter can employ other solid state semiconductor-based switching devices such as silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), gate commutated thyristors (GCTs), or other switching devices. Further, in at least some embodiments, the rectifier 8 can simply employ diodes.
As the switching devices are activated in the rectifier 8 and/or the inverter 10, time-varying common-mode voltages are produced. The common-mode voltages appear in the output phases A, B, and C of the VSI power converter 2, and hence, on the motor windings of the motor load 6. Where a neutral (shown in FIG. 1 as a node g) associated with the input power source 4 is grounded, the common-mode voltage appears between the motor windings and ground. Assuming that, as shown, the motor windings of the motor load 6 are coupled together, the common-mode voltages appear as an overall common-mode voltage Vo at a node 0, which can be understood as being coupled to actual ground by way of a stray capacitance Co. While the common-mode voltages can include multiple components at different frequencies, commonly the predominant or largest component of a common-mode voltage is at three times the source frequency. Thus, for a typical three-phase AC source providing 60 Hz power, the predominant or largest component of the common-mode voltages that are generated by the power converter 2 will be at about 180 Hz.
Depending upon a variety of factors including the power levels involved, environmental or other operating conditions, and the design of the motor load 6 (or other load), common-mode voltages in some circumstances can become fairly high in amplitude and, in any event, can potentially reach levels that are beyond the insulation rating(s) of the motor windings (or other load rating). Consequently, running of the motor load 6 in a manner resulting in such common-mode voltages can risk damaging the motor or decrease its life expectancy. Further, although motors can be designed with high insulation ratings such that the motors will not be harmed by such common-mode voltages, not all motors are so designed and increasing the insulation ratings of a motor can increase the cost of a motor. Additionally, it would be desirable for motor manufacturers if they did not have to take into account the risks posed by common-mode voltages when designing their motors.
Given that it would be desirable to have power converters that did not generate common-mode voltages, or at least only generated common-mode voltages that were significantly reduced in comparison with those generated by conventional power converters, a variety of modified power converter designs have been developed or attempted. Yet each of these modified power converter designs either fails to reduce common-mode voltage levels to satisfactory levels or introduces other disadvantages. For example, some modified power converter designs attempt to reduce the levels of common-mode voltages that they generate by more carefully controlling the pulsing on and off of the IGBTs or other switching devices within the power converters. Yet such modified power converters are more complicated to operate and control, achieve their results at a cost to the power converters' modulation indices and/or THDs (total harmonic distortion levels) of voltage and current, and in any event fail to eliminate the common-mode voltages.
Other modified power converter designs attempt to compensate for and nullify the common-mode voltages by producing anti-common-mode voltages through the use of additional switches, or through the use of isolation transformers. However, the use of additional switches can increase the complexity of controlling operation of the power converters, and can increase the cost of the power converters by increasing the number of circuit components. Further, where transformers are employed, the transformers must be rated to handle the common-mode voltage levels. Also, the use of such transformers increases the cost of the power converters and, due to the size of the transformers, can increase the bulkiness of the power converters.
Still additional modified power converter designs employ additional passive L-C filters to suppress the common-mode voltages. The filters typically include both inductors to block the high-frequency common mode voltages and capacitors to shunt the high frequency common-mode voltages to ground. Conventionally, three-phase AC L-C filters are positioned between the inverter of the power converter and the load, e.g., three different L-C filters are implemented as part of (e.g., in series with) the output nodes 38, 40 and 42 between the IGBTs 32, 34 and 36 and the load 6 shown in FIG. 1, in association with each of the different phases A, B, and C. While the use of such filters does succeed in reducing the levels of common-mode voltages, the filters do not necessarily achieve desired reductions, particularly insofar as the filters associated with each of the different phases A, B, and C do not necessarily operate in a collective manner that might address imbalances between the common-mode voltages at the different phases A, B, and C.
For at least these reasons, therefore, it would be desirable if an improved power converter could be developed that generated reduced levels of common-mode voltages than conventional power converters, or even entirely eliminated such common-mode voltages. Further, it would be advantageous if such an improved power converter did not require significant numbers of costly additional components, require physically large or bulky components, and did not require more complicated control techniques to be implemented in relation to controlling the operation of its switching devices or other components. In at least some embodiments, it would be advantageous if such an improved power converter could serve as an improved drive that was capable of providing improved three-phase AC power with reduced levels of common-mode voltages (or no common-mode voltages) to three-phase AC motors or similar machines.